1. Field of the Invention
The present invention generally relates to improved methods for completing wells formed in both consolidated and unconsolidated formations, and for completing deviated wells formed in consolidated formations which require stimulation by hydraulic fracturing. More particularly, the present invention relates to more efficient and less expensive methods for well completion in underground reservoir rock formations comprising novel techniques for the utilization of permeably consolidatable, resin coated packing materials.
2. Description of the Prior Art
Increased emphasis is being placed on proper initial well completion as the value of nonrenewable natural resources increases and the costs associated with their production escalate. While such emphasis is especially felt in the areas of hydrocarbon extraction, production of other valuable liquids, such as groundwater, also emphasizes the need for low cost, efficient production techniques. Maximum reliability and productivity of any production program is essential, particularly offshore and in remote locations.
The aforestated general objectives are difficult to obtain where the reservoir rock is unconsolidated or otherwise subject to failure, or where the reservoir requires stimulation to make production or injection economically attractive. Extraction from such formations usually require the utilization of techniques cumulatively referred to as "sand control". Alternatively, such extraction may require hydraulic fracturing, or the combination of the two. The sand control mechanism, however, is exceedingly complex and is influenced by every well operation from first bit penetration throughout the lifetime of production or injection of the well.
Sand problems are most common in younger Tertiary sediments, particularly of the Miocene epoch. Notable examples are extensive, troublesome sand production areas in such sediments in the U.S. Gulf Coast, the Los Angeles basin of California, Canadian tar sands, Indonesia, Nigeria, Trinidad and Venezuela. However, sand inflow also occurs in other formations (i.e., older tertiary) if existing in situ stresses are altered by drilling, completion and production operations such that the rock matrix is weakened, thus allowing movement of sand into the wellbore, casing and tubing.
Factors contributing to the onset and continuance of uncontrolled sand production are numerous. One common basis is alteration of the stresses on the reservoir rock. If the balance of forces on the reservoir rock are sufficiently unbalanced, destruction of the rock matrix generally will follow.
Sand flow from unconsolidated or failure prone consolidated formations is often controlled through chemical or mechanical means to prevent or correct various problems including premature failure of artificial lift equipment; production loss caused by sand bridging in casing, tubing, and/or flow lines; failure of casing or lines and formations damage near the wellbore due to removal of surrounding formation, compaction, and erosion; abrasion of downhole and surface equipment; and handling and disposal of production formation materials.
A variety of techniques have been developed in the art to address the above noted problems of sand flow. One such method involves the process of injecting chemicals into the naturally unconsolidated formation to provide in situ grain-to-grain cementation Techniques for accomplishing this successfully are perhaps some of the most sophisticated undertaken in completion work. In closely related methods, sand or other appropriate matrix particulates are treated chemically and then injected into the wellbore (or through the perforations if casing is set), and into the formation where the resulting "pack" consolidates. Production is then commenced through a slotted or perforated liner or casing which is run along the length of the production zone. This technique is commonly referred to as resin coated sand gravel packing, or alternatively, resin coated sand consolidation.
In the consolidated gravel packing art, a particulate, usually a round silica sand of appropriate size and density, is coated with a suitable oil or water based epoxy or plastic resin and placed in a suitable carrier to form a viscous slurry. Such a resinous particulate slurry is described, for example, in Copeland, et al., U.S. Pat. No. 4,074,760. This slurry is then injected into the formation through the work string and perforations in the casing to form an area of high mechanical strength and high flow conductivity immediately adjacent the production inlets in the casing. Alternatively, a resin coated particulat may be placed between a wire wrapped screen or slotted liner and the casing.
Standard techniques for the application of such viscous particulate slurries may be described as follows. Once a well is established in the reservoir zone, the wellbore may generally be completed by one of two methods, either cased hole completion or open hole completion.
The general sequence of performing a cased hole completion includes drilling the hole, setting and cementing casing in place, perforating the casing for production; cleaning the perforation of damage and debris by flowing back, washing the perforations using a perforation wash tool, surging or perforating underbalanced; and stimulating the formation to decrease the skin factor as needed to make the well economically attractive. If the formation requires sand control, this is an additional step which must be performed in addition to the above.
The typical cased hole completion sequence further contains some or all of the following elements. The borehole is drilled with mud as the well fluid leaves a region of impaired or damaged permeability adjacent to the borehole. Casing is next run and cemented in place using a Portland cement slurry. Use of such a cement slurry may cause further damage to the formation's native permeability adjacent to the wellbore. However, The Portland cement functions to mechanically support the casing and also to isolate individual permeable formation from each other. At this point, the drilling mud is exchanged with a clear, solids free, completion fluid to reduce the probability of severe damage to the formation permeability during the critical completion phases where the formation is not protected from incompatible fluid invasion by the protective filter cake of mud solids. The casing is next perforated, and a method of cleaning the crushed formation and perforating debris is used to clean and open the perforations. The perforation cleaning steps may consist of washing the perforations using a perforation wash tool, backsurging the perforations, or by underbalanced perforating. The latter method is now considered by most operators to be the most effective. Finally, the appropriate hardware can be positioned in the wellbore for either a RCS consolidation, a gravel pack, or a conventional completion without any sand control.
The general sequence of performing an open hole completion includes drilling a pilot hole through all formation down to the top of the deepest target formation; setting and cementing casing in place; drilling a pilot hole through the deepest target formation; opening the hole size of the formations to be completed using an underreamer to remove damage from prior operations; and stimulating the formation to decrease the skin factor, if needed, to make the well economical. Again, sand control measures are performed in addition to these steps.
The typical open hole completion further comprises the following elements. The final borehole through the interval to be completed is preferably drilled with a nondamaging drilling mud. Casing is then set above the productive interval. If the interval to be completed has not already been drilled though, a pilot hole is drilled through the interval or intervals to be completed. If the pilot hole is drilled with a damaging drill mud, then the hole must be underreamed through the completion internal to remove as much as possible of the damaged formation adjacent to the wellbore. This operation is normally performed with a underreamer rotated on a workstring of pipe, and using a non-damaging circulating fluid to carry cuttings to surface. Once the desired sections of formation are exposed, they may be isolated from each other using inflatable cement packers, or, alternatively, the isolated sections of hole may be cemented using formation packers and port collars. If sand control is required, normally a gravel slurry is placed between a slotted liner or wire wrapped screen and the exposed formation, again using ported collars and a combination tool. The combination tool functions to open and close the port collar and also isolates the port collar to direct the slurry placement. Once each exposed formation section is treated in this way, sand control is accomplished and the well is ready to produce.
The production rate possible from the above described completions techniques may be enhanced by stimulating the well. Well stimulation may consist of a chemical stimulation using some kind of acid or solvent solution to dissolve or remove material from the formation. Alternatively, the well may be stimulated by hydraulic fracturing. In hydraulic fracturing, a fracture is created in the reservoir rock by hydraulic forces and then propped open by a particulate material. This propped fracture provides a low resistance flow channel from deep within the formation to the wellbore.
The above described methods of well completion have a number of disadvantages. Present methods of completing unconsolidated or failure prone consolidated formations using the combination of a hydraulic fracturing method and some type of sand control have not yet addressed the problem of having both fracture entry control and the high perforation density needed for a high efficiency completion. If the fracture stimulation is performed with a limited number of perforations to achieve fracture propagation throughout the entire interval, then the well must be produced at low efficiency through the limited number of perforations. If the interval is re-perforated after the fracture stimulation, several problems arise. First, there is no guarantee that most or all of the second set of perforations will connect to the propped fracture. Therefore, the increased flow capacity of the fracture will be choked back at the wellbore since the fracture will connect to only some of the perforations.
Second, operations employed to clean the second set of perforations by flowthrough or surging may cause loss of proppant from the fracture, resulting in closure of the propped fracture at the critical wellbore juncture. Third, the high flow density at the interface between the fracture and the wellbore represents a potential problem area for both fines movement and plugging, as well as problems arising from deposition of organic deposits. The high flow velocity at the interface causes the fines to migrate and the large pressure drop at this point tends to precipitate out any paraffin or asphaltene deposits where they give the most restriction to production. A further problem with such high velocity fluid movement through the sands involve the possible erosion of the plastic bonds between the particulates. This problem of erosion is particularly acute for zones close to the wellbore and subject to multiphase flow.
Yet another disadvantage of conventional methods of completion utilizing contemporary techniques for sand control involve the time necessitated in securing the casing along the production zone and in isolating separate reservoirs, and further, in the numerous subsequent completion steps required to complete a number of narrow production intervals in such zone. In such cases, cement seals or formation packers must be introduced between each production interval. Then formation damage or debris must be removed from the interface between the virgin, undamaged formation and the wellbore to effect an efficient completion. Ordinarily, this removal must be accomplished prior to placement of sand control measures for each formation interval to be completed. This debris removal is generally accomplished via chemical means, mechanical means, or a combination of the two.
Underreaming is generally the preferred mechanical method for damage removal prior to placement of sand control measures in open hole completions Underreaming, however, is highly time consumptive since it entails the introduction of the underreaming tool via a work string, hence necessitating at least one "round trip" for each production interval. Furthermore, additional trips may be required if hole stability problems are encountered. Even when the production intervals are grouped in a single zone, such damage removal processes can involve extensive time expenditures. Such time expenditures are especially noteworthy in deeper wells or in deviated wells having a large net length. Additionally, underreaming procedures conducted in some unconsolidated formations might result in a complete collapse of the borehole, and hence abandonment of the well.
Further disadvantages of contemporary completion art involve the inconsistency in the character of the high conductivity region created by the resin coated particulate slurry. Contemporary completion techniques incorporating provisions for sand control describe the introduction of the resin particulate slurry into the wellbore in such a fashion as to cause laminar flow of the slurry in the annulus between the wellbore and the production casing. If the casing is situated off center in the wellbore, the laminar flow of the resin slurry often leaves unfilled voids immediately adjacent the casing which may often decrease production efficiency. Such problems are again particularly acute in deviated wells where undesirable "duning" occurs. Laminar flow of such sands into the annulus also allows time for the formation to dehydrate the resin slurry, also resulting in premature and often unsatisfactory set up of the consolidated formation adjacent the production casing.